This would make him the third biggest investor in Twitter, with a greater stake than new Twitter CEO Jack Dorsey, who owns around 3 percent. The San Francisco-based microblogging company's biggest shareholder is co-founder Evan Williams, who owns around 6.8 percent.

A Twitter spokesman confirmed to me that the account was verified as belonging to Ballmer. The spokesman could not confirm the investment, however. Ballmer wasn't immediately available for comment. Bloomberg reported that Ballmer had confirmed his investment, which would amount at current valuation to be around $840 million.

His new Twitter account shows a fairly conservative spread of interests.

Of the 24 people he's following, four are Clippers players, one is LeBron James and another is Dallas Mavericks owner Mark Cuban. The remainder include many famous names, such as golfer Jordan Spieth, Microsoft co-founder Paul Allen and the White House.

Those of a mischievous mind might also observe that there's one famous name he isn't following: Bill Gates.